1. Field of the Invention
The present invention relates to an industrially advantageous preparation of .beta.-isopropylnaphthalene hydroperoxide, and more particularly to an industrially advantageous process for preparing .beta.-isopropylnaphthalene hydroperoxide by oxidizing a mixture of isopropylnaphthalene isomers containing .beta.-isopropylnaphthalene as a main component with oxygen or air, and recycling unreacted isopropylnaphthalene which is recovered at any step after the oxidation to the oxidation zone as a material for oxidation.
2. Description of the Prior Art
It is well known that .beta.-naphthol is obtained by oxidizing .beta.-isopropylnaphthalene (isopropylnaphthalene being hereinafter referred to as "IPN" for brevity) with oxygen or air and then subjecting the resulting .beta.-isopropylnaphthalene hydroperoxide (isopropylnaphthalene hydroperoxide being hereinafter referred to as "NHPO" for brevity) to a catalytic cleavage in the presence of acidic catalysts.
The .beta.-IPN, the material for oxidation is generally prepared by alkylation of naphthalene with propylene, and in the alkylation, .alpha.-IPN is always produced as a by-product under any conditions. Therefore, the material for oxidation is not pure .beta.-IPN and, in general, contains a certain amount of .alpha.-IPN.
The oxidation of IPN is generally carried out by blowing molecular oxygen or an oxygen containing gas through an oil containing IPN, while the mixture is kept neutral to alkaline. On investigation of the oxidation process, it has now been found that all of the IPN, particularly the .beta.-IPN contained therein, is practically impossible to oxidize completely for the reason that an increased conversion of IPN is followed by a reduction in oxidation rate of the remaining IPN whereby the total reaction time is prolonged, and that the prolonged reaction time (1) causes side-reactions to occur, for example, a conversion of the NHPO produced into a carbinol-type compound, thus remarkably reducing the yield of the NHPO, (2) allows the reaction system to become increasingly viscous thus making the handling of the reaction liquid increasingly difficult, and (3) at last leads to a serious problem from the standpoint of safety, such as handling of a very reactive highly concentrated hydroperoxide. Thus, it is most advantageous to stop the reaction when the NHPO concentration has reached about 10 to 40% by weight, from the standpoints of reaction time, yield of NHPO, handling of the reaction liquid and safety.
It is however very uneconomical due to a large loss of the starting material to carry out the oxidation with such a relatively low conversion and moreover without re-using the unreacted starting material, and therefore it is essential in an industrial scale production to recover the unreacted IPN after the oxidation and to recycle the unreacted IPN to the oxidation zone for reuse as a starting material.
Repeatedly recycling the unreacted IPN recovered results in a remarkable reduction in oxidation rate of .beta.-IPN (consumption rate of .beta.-IPN) which would not be expected from an oxidation using IPN which is prepared from naphthalene and propylene without use of the unreacted IPN recovered, so that the selectivity of .beta.-IPN to .beta.-NHPO (yield of .beta.-NHPO based on the reacted .beta.-IPN) is markedly decreased increasing the amount of by-products and decreasing the yield of .beta.-NHPO and the quality of the .beta.-naphthol as a final product. That is, it is very difficult to repeatedly recycle the unreacted IPN recovered to produce .beta.-NHPO in an industrially advantageous manner where the IPN recovered is used as it is as a material for oxidation.
Research on the oxidation process has been conducted in order to overcome this difficulty, and it has now been found that a main factor hindering the oxidation is the presence of .alpha.-IPN, and further that .beta.-NHPO can advantageously be prepared without a reduction in oxidation rate and selectively to .beta.-NHPO by controlling the .alpha.-IPN content of the IPN mixture fed to the oxidation zone to about 15% by weight or less based on the weight of the IPN mixture.